UCMP MYSTERY FOSSIL #24 is . . .Conularia

Conulariids have at times been classified as coelenterates, worms, gastropods, cephalopods and hemichordates though there really isn't enough evidence to assign them to any of these groups. They tend to be pyramidal in shape with the sides being covered in fine transverse ridges (as in this example). Conulariids may have been attached to the sea floor early in life and then free swimmers later on. Find out more about conulariids here.